BIRTE JUNGE1, GERD DERCON2, ROBERT ABAIDOO1, DAVID CHIKOYE1, KARL STAHR31International Institute of Tropical Agriculture (IITA), Nigeria2International Atomic Energy Agency (IAEA), Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Austria3University of Hohenheim, Institute for Soil Science and Land Evaluation, Germany
Soil erosion is one of the most critical environmental problems in sub-Saharan Africa. It causes on-site degradation of the natural resource base, as well as off"=site problems. Reduction of soil loss is therefore important to maintain soil productivity and contribute to food security in the region. Quantitative data on the extent and rates of soil redistribution are necessary to guide the development of effective resource management. The use of fallout radionuclides as tracers can provide the required information, since fallout radionuclides are quickly and strongly adsorbed by fine soil particles after deposition and primarily redistributed by physical processes associated with water and wind.

The contribution represents the test of this technique conducted in Ibadan, Nigeria, (730'N 354'E) in 2007 and 2008. To describe the spatial and vertical distribution of caesium-137 (137Cs) in undisturbed soils, reference sites were sampled at the beginning. Fields characterised by ridges prepared parallel to slope or by flat bed preparation were sampled according to a grid design to determine the spatial 137Cs distribution. Cores for depth-incremental sectioning were also taken on the upper and lower slope to describe the 137Cs depth profiles. All soil samples were analysed by gamma spectrometry using a high"=purity Germanium (HPGe) detector. The results obtained from the reference sites show the highest 137Cs concentration in the upper few centimeters of the topsoil and a decrease with depth. The analyses of the samples collected from the field are still in progress. As soil erosion was observable, it is expected that the 137Cs inventories will be reduced in the soil of the upper slope and increased in the deposition zone on the lower slope. Different conversion models, including mass balance models, will be used to estimate the rates of erosion and deposition based on the measurements. The 137Cs inventories will be interpolated to visualise the spatial distribution of soil redistribution within the study area on a map. Hence, the study will provide quantitative data on soil redistribution in the savannah of West Africa and contribute to improved soil conservation in Africa.